siphonage from trap under bathroom sink

[samwj]

The trap under my bathroom sink is emptying each time we flush the toilet or put water down any of the drains in our flat. We live on the first floor above garages that do not have water supplies so ours is the only waste going into the soil stack, and all at the same level. We have taken the AAV off the top of the soil stack in the loft in an attempt to allow air into the stack and stop it siphoning water from the trap, but it has made no difference. Am now considering a self sealing valve (hepvo) for under the sink, but would like to know if there is anything else we should try first.

It's a new property and we were the first to live here, we moved in 4 years ago and this problem only started recently, we have not done anything different recently to make this happen.

I am only a woman who does not understand plumberspeak so please could you make any suggestions in plain english so I can understand them!!
ANY HELP PLEASE !!

[plumbbob]

The easiest solution is to replace the basin trap with an anti-vac trap or if this is difficult, fit a small aav in the line from the basin to the soil stack.

[samwj]

Thank You for your advise plumbbob, I will most likely replace the trap under the sink as all the pipework from the sink disappears behind tiles and is boxed in along the wall and under the inaccessible end of the bath. Got to admit that I find plumbing scary 'cos I tend to sort out one part and create a problem somewhere else. Here comes a weekend of pain and cursing!